For a given level of insulation (illumination intensity), a photovoltaic (PV) energy source may be characterized by a graph of current versus voltage, generally referred to as the current-voltage (I-V) curve. It is known that when the PV source is uniformly illuminated, then such a PV source typically has one unique value of current and voltage at which maximum electrical power can be extracted for a given illumination intensity and/or temperature.
To extract maximum electrical power, an electrical load connected to the PV source must be adjusted such that the I-V curve of the load intersects the I-V curve of the PV source at the maximum power point. This is commonly achieved by coupling to the PV source an active load, such as a switching power converter, controlled to dynamically seek the maximum power point of the PV source by adjusting its respective I-V characteristics as a function of sensed PV source characteristics.
The switching power converter, also referred to as the PV load converter, may be configured to output useful electrical power, which may be processed by additional converters or supplied directly to the working load. The power of the PV source may be monitored by a suitable power monitor means, and a maximum power point tracking algorithm may then be processed in a suitable processor to determine how to dynamically adjust the switching converter operation such that its input I-V curve intersects the I-V curve of the PV source at the maximum power point. A generated control signal resulting from the processing of the algorithm is fed to the converter so that it may be adjusted in such a manner. In this setup it is generally presumed that the working load will use essentially all of the available power of the PV source.
A PV system equipped with maximum power point tracking should, in theory, be able to dynamically track changes due to variations in the environment of the PV source and/or due to aging of the PV source and should extract maximum electrical power during the operation of the PV system. In practice, however, there are some issues concomitant with maximum power tracking techniques. First, the implementation of the tracking algorithm in a processor requires an incremental consumption of electrical power for powering such a processor, thus reducing harvesting efficiency of the PV system. Second, maximum power tracking is generally much more complex than just regulating the switching converter to a reference and the switching operation of the converter can interfere with appropriately determining the maximum power point and, in some cases, is prone to operational instabilities while seeking to solve the maximum power point algorithm. Third, power tracking algorithms commonly require sensing of both current and voltage of the PV source. This current sensing often results in additional losses, thus further reducing the overall efficiency of the PV system.
Thus, it would be desirable to provide a photovoltaic system not subject to the foregoing issues. It would be further desirable to use any of various relatively low cost and reliable PV load converter topologies with I-V curve characteristics suitable for combined integration (e.g., power stacking) of multiple PV sources, thereby leading to efficiencies of scale, such as may be conducive to constructing a large PV array with any desired number of basic photovoltaic modules.